Over time mattresses can sag and feel too soft to be comfortable, especially for individuals with back problems or injuries. For a temporary fix to firm up a soft mattress, you don't need to spend a lot of money on expensive mattress replacements when a simple piece of plywood may offer the adjustment you need for a good night's rest on a firmer bed.
Measure the width and length of your mattress. Purchase a sheet of 3/8-inch plywood according to those measurements or cut a sheet of plywood to fit. Most home improvement stores that sell plywood will do simple cuts for free. If not, mark the mattress measurements on the plywood sheet with a pen or pencil, then saw off excess materials using a handsaw or circular saw.
Slide the mattress off the bed and place it on the floor. Place the plywood sheet directly on top of the mattress foundation or box springs. Adjust the piece to fit so no edges show from underneath the mattress.
Place the mattress on top of the plywood sheet. Sit or lay on the mattress to test for firmness.
If using a queen- or king-size bed and only one person needs to increase firmness, adjust the measurements of the plywood to fit under only one side of the bed. To do this, simply divide the measurement of the mattress width in half and cut the plywood to fit. Always wear protective eye wear when sawing wood pieces or operating power tools.